Taciturn: habitually non-talkative.
This prompted me to think about the definition: how can something which is the lack of an action be habitual? Habitually forgetful, I would understand, would be an active action whereby you actively learned and then forgot. Habitually non-talkative, though, doesn't seem like it can become a habit.
Update: Reference.com's word of the day uses the phrasing "Habitually silent". In the sense that silence can be enforced, can it be habitual?